The invention relates to the protection of direct current electrical loads from damaging overcurrents. More particularly, this invention relates to protection against fast transients like those caused by discharge of system high voltage capacitance elements.
Electrical loads that are connected to high-voltage d-c power supplies are frequently subject to a risk of destruction by delivery of some of the power from the supply to portions of the load in excessive quantities as a result of a fault. This is especially true of high-voltage accelerating structures connected to high-voltage power supplies, although almost any load that is connected to a d-c power supply is capable of being damaged at least locally, by fault currents. However, the threat of electrical breakdown is greatest in those electrical loads that are connected to power supplies of relatively high voltage, of the order of tens of kilovolts. One arrangement to protect against transient overloads was described in U.S. Pat. No. 4,054,933, issued to the inventor of the present invention on Oct. 18, 1977. In that patent, a saturated time delay transformer connected in series with a load detected a load fault and limited the fault current to a safe level for a period long enough to correct the fault or else disconnect the power supply from the load.
Frequently, a large fault energy is stored in the system leakage capacitance to ground. One arrangement for limiting system capacitance discharge currents includes an arc snubber device in which single turns of several conductor pairs (each comprising separate discharge paths) are passed through a magnetic toroidal core of 50/50 NiFe material, such as Deltamax. This material is much more expensive than common silicon steel magnetic materials and presents unique problems as will be explained below. The single turns which pass through the interior of the toroidal core are arranged as bifilar windings, i.e., windings comprising complete sets of bipolar conductors which form a complete direct current circuit when connected between a load and its source. Such snubber devices interpose high resistance (caused by eddy current losses) between the capacitance discharge source and the load. Deltamax material is usually used for the core since it is especially effective for providing eddy current resistance. Such arrangements require a magnetic reset winding or circuit which returns the Deltamax core after cessation of the magnetizing fault current, to a point on the core's magnetization or hysteresis loop operating curve where the residual field in the core is very small. Such reset circuits have iron losses which contribute to the fault current through the load.
The Deltamax toroidal cores provide an exponential RC discharge of the system leakage capacitance. Inherent in such circuits, due to their iron losses, is an undersirable initial current step which allows a large I.sup.2 t discharge through the system to be protected. The energy peak associated with a large initial I.sup.2 t value is very destructive since it causes metal vaporization on occasion. At the present time, snubbers are limited by a maximum operating voltage, about 200 KV. Despite design theory, snubbers are unable to limit fault current peaks in systems operating above about 200 KV according to W. R. Baker and D. B. Hopkins in their paper entitled, "Present and Future Technology of High Voltage Systems for Neutral Beam Injectors," dated Jan. 27, 1978. Further, snubber arrangements are very expensive and heavy, so as to be unfeasible for many retrofit improvements for existing high voltage systems. The cost and size of the toroidal snubber arrangements can be more readily understood with regard to the following relationship which describes the initial transient fault value, or current step i.sub.s, of such a snubber as: EQU i.sub.s =kl(E/WNn.sup.3).sup.1/2
where k is a constant related to the physical property of the core material (3.18 for 50/50 NiFe; 10.5 for Silectron steel), 1 is the mean circumferential length of flux path in the toroidal core, W and N are the width and total number of core laminations, respectively, n is the number of turns of the coil, and E is the step voltage applied to the snubber coil. Systems having a high voltage (several hundred kilovolts) and high current (several thousand amperes) load can accommodate only a single turn (n=1). The very large cross-sectional area of the coil conductors needed for large current handling capability effectively prohibits multiple turn coils which must be disposed in the interior of a toroidal core. Also, the distance between conductors of adjacent turns of a coil must be large to accommodate the high voltage levels. Therefore, in order to put more turns through the interior of a high energy toroidal core, the inner diameter of the core must be increased, causing a proportional increase in the length 1 of the flux path, and in the step current, according to the above relationship. Because of its magnetic properties, Deltamax material is usually used, as opposed to increasing the number of turns n, which is not possible in the high energy applications as noted above. The magnetic properties of the Deltamax material, particularly its magnetic permeability, changes with applied mechanical stress. Therefore elaborate precautions must be taken to protect the Deltamax material from stress, e.g., each core section of the material is encased in a nylon sheath filled with rubber. Further, after winding, the core must be annealed in an inert atmosphere, to eliminate winding stresses and to achieve the desired magnetic properties.
In operation, such snubber devices, while lowering the amount of I.sup.2 t consumed during fault conditions, allowed an initial step or fault current through the system they protected. This steeply rising current wave presents dangerous I.sup.2 t levels to load portions of the system. The snubber causes the initial step current to decay exponentially (RC discharge) through the equipment to be protected. This current decay pulse may damage the equipment to be protected.
It is therefore an object of the present invention to provide a device for limiting capacitive discharge current, which has a reduced size and weight and which is comprised of a fewer number of inexpensive parts.
A further object of the present invention is to provide a current limiting device which reduces initial step current increases and which decreases the initial rate of rise of the fault current, thereby reducing the overall let-through I.sup.2 t value fed to the load.
Another object of the present invention is to provide an oscillatory discharge circuit which limits fault current discharged from the circuit leakage capacitance through the load, to a slowly rising L-C discharge which has a small initial current value, and has a peak current which can be controlled by a practically achievable appropriate inductance.
Another object of the present invention is to provide a protection device having a magnetic core that is constructed of inexpensive material and does not require magnetic biasing or reset circuits.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.